This view was supported by Allison and Cicchettie (1976) who found that larger animals had less NREM sleep, because they don’t need to conserve as much energy as those with a high metabolic rate. This shows that it is NREM sleep that is important for energy conservation and can therefore partially contribute to the explanation of the function of sleep.
On the other hand, it has been found that sleep does not conserve a great deal more energy than when resting, sleep reduces energy rates by 5-10%. This suggests that rest would be just as adaptive as sleeping. Further criticism would suggest that it would be safer for prey animals to rest and not sleep, as it does not come with certain dangers that sleep has, such as loss of consciousness, which further endangers the animals. This means that the “energy conservation” explanation is not sufficient to explain the function of sleep. Aswell as this, the unilateral sleeping habits of marine mammals and some birds also indicates that the sleep function can be fulfilled while an animal is still partially awake. This suggests that conserving energy cannot be the key feature of sleep. It may be that inactivity serves another purpose such as lowering core body temperature and not energy conservation.
Another explanation for the function of sleep is the restoration perspective. Oswald believed that the function of sleep was to re-group our resources after expending them all during the day. In particular, he believed that REM sleep was important for brain growth and repair whereas slow wave sleep (SWS) was important for bodily growth and repair.
In order to support this theory, Oswald observed patients who were recovering from injuries to their central nervous system (CNS). He found that there was a significant increase in the amount of REM sleep and suggested that this was suggestive of recovery processes going on in the brain. This supports the theory of restoration, signifying that in a time where severe recovery is needed, the brain will allow more REM sleep to occur in order to speed up recovery. This has strong implications in the function of sleep.
However, this is a correlational study, which means that we cannot infer cause and effect. We don’t know if the increase in REM sleep was because the brain needed to recover or if it was simply just caused by the injury to the CNS. It may even be due to a third factor, which may have caused both the injury and the increased REM sleep.
The findings are also questionable because the participants were only those who had suffered from injury to their CNS, which means that we do not know if we can generalise it to perfectly healthy people who have not had an injury. It may just be that only people with injuries to their CNS use REM sleep as a means of recovery. Overall, this research can only, at best, make limited contributions to the explanation of the function of sleep, as we do not know if we can apply the findings to everyday life.
On the other hand, a study by Adam and Oswald (1983) found that tissue growth of new cells in the skin takes place more quickly when we are asleep. This is strong research to suggest that restoration does occur and seems to provide a more valuable contribution to explaining the function of sleep.
Oswald also suggests that sleep is important for conserving and replenishing biological substances such as neurotransmitters and hormones. For example, over the course of the day, our neurotransmitter levels fall. During REM sleep, some neurons synthesize new neurotransmitters for release during waking. This suggests that we sleep in order to replenish key brain chemicals in order for us to be able to function normally again once we are awake.
This is also true of hormones and SWS. SWS is related to the secretion of growth hormone, which stimulates protein synthesis to take place. This reaction is fundamental in the re-growth and repair of body tissues because proteins are fragile and must be continuously renewed. This continuous restoration of essential proteins is part of the body’s natural healing process. For growth hormone to be secreted, uninterrupted SWS is vital. This also suggests that sleep is imperative for the body to function properly, providing a valuable explanation for the function of sleep.
In order to test the theory that sleep is necessary for the restoration of the bodily processes, many studies of sleep deprivation were carried out, most of which split into two categories. Studies were either of total sleep deprivation or partial sleep deprivation.
One case study consisted of an American DJ named Peter Tripp who stayed awake in a 201 hour “wakeathon”. It was found that, 3 days into the experiment, Tripp became abusive. After 5 days, he claimed he began to hallucinate (for example, he kept seeing spiders in his shoes and mice and kittens in random places) and became extremely paranoid (he accused people of drugging his food and accused a technician of putting a “hot electrode” in his shoe). By the end of the study, his body temperature had dropped and his waking brain wave patterns were practically identical to sleeping brain waves. It was then reported that, after 24hours of sleep, he awoke and felt fine.
This shows that going without sleep, even for lengthy periods, does not result in long term damage according to self-report. There is also no indication that you need to recover anything like the amount of sleep lost. However, friends and family of Tripp said that after the stunt, he was never quite the same again, which may cause us to be unsure about the extent of damage that the stunt may have produced. It may just be that Tripp was psychologically affected by the lack of sleep and not biologically damaged or that the friends and family were affected by certain demand characteristics, which caused them to expect that Tripp had suffered some serious damage as a result of the experiment.
However, in 1965, Tripp’s record was smashed by 17-year-old Randy Gardner, who managed to stay awake for 264 hours (11 days) for a supposed science fair project. Dr. Dement and some of Randy’s high school friends kept him awake and tracked his condition by administering a series of tests, keeping him active at night by going for drives, playing loud music and long games of basketball and pinball. Unlike the study with Tripp, the one thing they didn’t do was give him any drugs, not even caffeine. To make sure he wasn’t causing himself brain damage or otherwise injuring his health, his parents made him get regular checkups at the hospital. The doctors found nothing physically wrong with him, though he did intermittently appear confused and disoriented.
However, there are many contrasting reports of Gardner’s condition during the experiment. According to some sources, as more days passed, Gardner suffered certain physiological symptoms of sleep deprivation e.g. speech began to slur, he had trouble focusing his eyes, he had trouble remembering what he said from one minute to the next, and some sources even state he, like Tripp, experienced hallucinations. Other sources state that he experienced no such symptoms and that he was physically and psychologically fine.
As of 2007, Gardner remains alive and well, having suffered no long-term ill effects from his experience, showing that sleep deprivation has no severe effects on the human body. This may suggest that sleep is not as vital for the body as was originally thought.
However, one limitation of these studies is that they are only studies of individuals. We do not know that the average human being would behave in the same way and so we cannot generalise this research to the rest of the population.
The research is also limited because a lot of the findings contradict each other. Some say that Gardner experienced no psychological symptoms at all and that Tripp did. Others say that Gardner did in fact experience problems, such as hallucinations just like Tripp. This means that we cannot justifiably generalize the findings to all human behaviour.
Further evidence to criticse this research comes from Williams et al. (1959) who found that, after 72 hours of sleep deprivation, it was impossible to prevent short episodes of microsleep while they are awake. EEG recordings show that microsleep was the same as sleep in terms of brain waves. This explains why Tripp’s EEG readings showed the same brain wave patterns as a sleeping person, suggesting that he was in fact getting micro amounts of sleep that were undetectable. This may explain why there were no long-term effects for the body and may suggest that sleep is as important to restoration as Oswald originally thought.
Overall, it seems that there are many contrary findings into the function of sleep. Who’s to say that sleep only serves one purpose? It may be that sleep serves as both an adaptive purpose for conserving energy and as a means of restoring the body and replenishing what has been lost throughout the day.